When you get a couple of strokes in but there's no money shot due to an inconvenient location or situation.
Wale: I was gonna fuck this girl, but my roommate had a girl in the room. So we had to go to the bathroom, but it was really cramped and we couldn't finish.
Dr. Dre: Oh so you got the half-body?
Wale: Yeah, pretty much The half-body sometimes can leave you with blue balls though.